Origin of the Moon

For a long period of time, the fundamental question regarding the history of the Moon was of its origin. Early hypotheses included fission from the Earth, capture, and co-accretion. Today, the giant impact hypothesis is widely accepted by the scientific community.
Origin of the Moon is explanations for the formation of the Moon, Earth's natural satellite. The leading theory has been the giant impact hypothesis.[1] However, research continues on this matter, and there a number of variations and alternatives.[1] Captured body, fission, formed together (condensation theory), planetesimal collisions (formed from asteroid-like bodies), and collision theories are some other ones.[2] However, most of these have a lot of problems which is one reason why giant impact hypothesis has been favored.[2]
GIH (Giant Impact Hypothesis) suggests a Mars-sized body called Theia impacted Earth, creating a large debris ring around the Earth which then formed the system.[1]
Something that needs explaining is why the Moon's oxygen isotopic ratios seem to be essentially identical to Earth's.[3] Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each solar system body.[4] If Theia had been a separate proto-planet, it probably would have had a different oxygen isotopic signature than Earth, as would the ejected mixed material.[5] Also, the Moon's titanium isotope ratio (50Ti/47Ti) appears so close to the Earth's (within 4 ppm), that little if any of the colliding body's mass could likely have been part of the Moon.
The most widely accepted explanation for the origin of the Moon involves a collision of two protoplanetary bodies during the early accretional period of Solar System evolution. This "giant impact hypothesis", which became popular in 1984, satisfies the orbital conditions of the Earth and M on and can account for the relatively small metallic core of the Moon. Collisions between planetesimals are now recognized to lead to the growth of planetary bodies early in the evolution of the Solar System, and in this framework it is inevitable that large impacts will sometimes occur when the planets are nearly formed. It is thought to have originated in the 1940s with Reginald Aldworth Daly, a Canadian professor at Harvard.
The hypothesis requires a collision between a body about 90% the present size of the Earth, and another the diameter of Mars (half of the terrestrial radius and a tenth of its mass). The colliding body has sometimes been referred to as Theia, the mother of Selene, the Moon goddess in Greek mythology. This size ratio is needed in order for the resulting system to possess sufficient angular momentum to match the current orbital configuration. Such an impact would have put enough material into orbit about the Earth to have eventually accumulated to form the Moon.
Computer simulations show a need for a glancing blow, which causes a portion of the collider to form a long arm of material that then shears off. The asymmetrical shape of the Earth following the collision then causes this material to settle into an orbit around the main mass. The energy involved in this collision is impressive: trillions of tons of material would have been vaporized and melted. In parts of the Earth the temperature would have risen to 10,000°C (18,000°F).
The Moon's relatively small iron core is explained by Theia's core accreting into Earth's. The lack of volatiles in the lunar samples is also explained in part by the energy of the collision. The energy liberated during the reaccreation of material in orbit about the Earth would have been sufficient to melt a large portion of the Moon, leading to the generation of a magma ocean.